This disclosure is directed at skeletal bone fixation systems, and more particularly to a fixation device and method for retaining vertebrae of a spinal column in a fixed spatial relationship.
Bone fixation systems are used to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments after surgical reconstruction of skeletal segments. Such systems may be comprised of bone distraction devices, skeletal bone fixation devices, bone screws and/or bone cables, and any additional instruments needed for implant placement.
Whether for degenerative disease, traumatic disruption, infection or neoplastic invasion, surgical reconstructions of the bony skeleton are common procedures in current medical practice. Regardless of anatomical region or the specifics of the reconstructive procedure, many surgeons employ an implantable skeletal fixation device to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during postoperative healing. These devices are generally attached to the bony elements using bone screws or similar fasteners and act to share the load and support the bone as osteosynthesis progresses.
Available systems used to fixate the cervical spine possess several shortcomings in both design and implantation protocols. These devices are manufactured and provided to the surgeon in a range of sizes that vary by a fixed amount. This mandates that a large number of different sizes must be made and inventoried—adding to cost for manufacturer, vendor, and end user (hospitals). More importantly, the pre-manufactured devices may not precisely fit all patients forcing surgeons to choose between a size too small or too large.
Current cervical systems are not modular, and will not permit addition of one fixation device to another for extension of the bony fusion at a future date. It is accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to adjacent levels. Currently, the fixation device must be removed from the spine and replaced with a longer device in order to extend the fusion to adjacent levels. This surgical procedure necessitates re-dissection through the prior, scarred operative field and substantially increases the operative risk to the patient. Further, since mis-alignment of the original device along the vertical axis of the spine is common, proper implantation of the replacement often requires that the new bone screws be placed in different bone holes. The empty holes that result may act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and lead to bone fracture and subsequent device migration.
Current systems may provide fixation that is too rigid. Since bone re-absorption at the bone/graft interface is the first phase of bone healing, fixation that is too rigid will not permit the bone fragments to settle and re-establish adequate contact after initial bone absorption. This process will lead to separation of the bony fragments and significantly reduce the likelihood of bony fusion. Unsuccessful bone fusion may lead to construct failure and will frequently necessitate surgical revision with a second operative procedure.
Benzel (U.S. Pat. No. 5,681,312) and Foley (patent application Pub. No. US2001/0047172A1) have independently proposed bone fixation systems designed to accommodate bone settling. In either system, however, bony subsidence causes one end of the device to migrate towards an adjacent, normal disc space. This is highly undesirable since, with progressive subsidence, the device may overly the disc space immediately above or below the fused segments and un-necessarily limit movement across a normal disc space. Clearly, accommodation of bone settling at the end of the fixation system is a sub-optimal solution.
The implantation procedures of current fixation systems have additional shortcomings. Distraction screws are used during disc removal and subsequent bone work and these screws are removed prior to bone plate placement. As is known to those skilled in the art, the distraction screws are mounted into the bone and used to separate the bones and provide access to the space therebetween. After the distraction screws are removed, the resulting empty bone holes created by removal of the distraction screws can interfere with proper placement of the bone screws used to anchor the device and predispose to poor alignment along the long axis of the spine. This is especially problematic since the surgical steps that precede device placement will distort the anatomical landmarks required to ensure its proper alignment, leaving the surgeon with little guidance during implantation. For these reasons, bone fixation devices are frequently placed “crooked” in the vertical plane and often lead to improper bony alignment.
The empty bone holes left by the removal of the distraction screws also act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and predispose them to bone fracture and subsequent device migration. Improper fixation device placement and bony fractures can significantly increase the likelihood of construct failure and lead to severe chronic pain, neurological injury, and the need for surgical revision with a second procedure.
While many vertebral fixation systems use bone plates, some systems employ longitudinal rods to connect and fixate the vertebra bodies. A number of these devices have been illustrated in U.S. Pat. No. 5,147,360, 5,152,303, 5,261,911, 5,380,324, 5,603,714, 5,662,652, 5,683,391 and 6,214,005. They share the shortcomings enumerated above and exhibit additional limitations of their own. Rod-based systems are usually larger and more bulky than plate-based systems, making these devices difficult to apply in regions with limited space such as the anterior aspect of the cervical spine. Further, these devices often require the assembly of multiple segments before implantation and are notoriously cumbersome to use. For those reasons, many surgeons will limit their use of rod-based fixation devices in general and avoid them altogether in regions with limited space, such as the anterior aspect of the cervical spine.
In view of the proceeding, it would be desirable to design an improved rod fixation system and placement protocol. The new device desirably provides the reliable bone fixation characteristic of rod-based systems as well as address the shortcomings enumerated above. The device is desirably of variable length and able to accommodate any length within a pre-defined range. It is desirably capable of accommodating bone settling at the level of bony subsidence and not encroach upon normal, adjacent disc spaces. The device desirably readily permits extension of the fusion at a future without requiring device removal. And, unlike prior art, the device desirably requires no intra-operative assembly, provides ease of use and is sufficiently compact so as permit application within the anterior aspect of the cervical spine.